Synchronization systems allow secure synchronization of data items among a number of devices without a central authority being able to gain access to the data items. A common way for a device to identify itself to others in the synchronization system is by using a public/private key pair, the private key of which is only kept on the device.
Over time, the set of devices that participate in synchronization may have a number of “ghosts” or non-participant members. These are devices that are valid members of the synchronization system that have lost their private key, making it impossible for them to participate again. One class of non-participant members is caused by devices that are members of the synchronization system and are erased and re-introduced back into the synchronization system. This could be because the owner selected to erase all contents and settings on a device (e.g., to lend the device to a family member temporally), or replaced the device's hard drive and decided to start afresh rather than restoring from a backup.
Non-participants cause performance and power issues, as other devices in the synchronization system continue to try to synchronize items with them. Non-participants might also clutter up management user interfaces and even cause a security problem. For instance, the owner of a device may become used to seeing old devices that are not used anymore being displayed alongside with the active devices. The owner might become less able to spot an attacker's device should one make its way into the synchronization system.